In order to design and/or improve the design of computer systems, it is desirable to know how existing computer systems behave. One behavioral aspect of computer systems that is desirable to know is the nature of bus activity, i.e., the nature of events that occur on a computer bus as data is being processed. Bus event information has become more desirable as computers have increased in complexity. It is particularly desirable in the design of multiprocessor computer systems.
In the past, computer software and hardware engineers have used indirect and inaccurate means for determining how a computer system is behaving. Software monitoring of computer system operation has been accomplished by adding performance monitoring code to the software being run on a computer system. Adding performance monitoring code has the disadvantage of modifying the system under test, thereby obscuring the true behavior of the system. Additionally, the amount of data developed by performance monitoring code has often been small and incomplete because rigorous performance sampling creates unacceptable degradation of computer system operation.
Computer hardware monitoring has been accomplished by the use of standard electronic instruments such as logic analyzers and laboratory counters. Unfortunately, neither of these devices are designed to distinguish and count a large number of complex events. Further, standard electronic instruments are very cumbersome to use and often require the attachment of a large number of test probes to the computer system whose activity is being monitored. While attempts have been made to produce specialized monitoring hardware, as a general rule, such hardware has only been able to distinguish and count a limited number of events. Further, it has been awkward to use.
Examples of previously developed software and hardware computer monitoring systems are described in U.S. Pat. No. 4,484,269 entitled "Apparatus for Providing Measurement of Central Processing Unit Activity," U.S. Pat. No. 4,485,440 entitled "Central Processor Utilization Monitor" and U.S. Pat. No. 4,590,550 entitled "Internally Distributed Monitoring System."
The present invention is directed to overcoming the foregoing disadvantages by providing a software-controlled bus event monitor that includes specialized hardware components designed to count a large number of different events that occur on the bus of a computer system without degrading the operation of the system.